The present invention generally relates to adaptive optics and, more particularly, to a segmented reflecting system and an actuator for providing position adjustment of individual optical elements of a segmented reflecting system.
Launch costs and payload volume restrictions currently prohibit sending extremely large reflecting mirror or antenna systems into space. Space-based systems also have stringent weight requirements because of the cost of sending a payload into space. There are several motivations, however, to develop these systems for defense and commercial telescopes and directed energy applications. For example, design of large and low-weight structures for optics and antennas is a primary technology to be developed for the U.S. Air Force (USAF), as identified by the USAF Scientific Advisory Board. In addition, the Jet Propulsion Laboratory (JPL) is studying proposals for a terrestrial planet finder space telescope, which is an example of a project that needs a lightweight mirror system that can provide massive light-gathering capacity, while allowing compact stowage in a spacecraft payload compartment. Even the relatively small Hubble space telescope mirror is very heavy at 200 kilograms (Kg). At a typical rate of $30 thousand per kilogram for launch costs, ground glass mirrors such as the Hubble mirror represent an inefficient solution for reflecting systems in space.
Thus, new methods to provide extremely large and accurate optical and antenna systems that accommodate launch constraints are needed. Some of the proposed methods include using low-weight materials. Design of single-piece structures with accurate geometry, however, is difficult with any material. Attaching reflectors to large foldable structures involves challenges in packing the foldable structures within payload compartments. Another problem with foldable structures is that there must be components within the design that function just to allow the foldable structure to be collapsed for launch and deployed in space. The components enabling this add cost and weight without providing functional value.
Free-flying mirrors have also been proposed that can be adaptively controlled for positioning in space. These segmented systems with their large reflecting surfaces, however, entail design challenges similar to those of any large system so that the practical assembly of these huge reflecting systems would still require launching large structures. An additional proposal includes making adaptive membrane mirrors. An obstacle to this approach is that it is difficult to correct local aberrations in the membrane surface without affecting the form of the surrounding reflecting area.
Typical space-based telescope or antenna systems include radiation sensitive components attached to a supporting structure. The radiation sensitive components can be optical devices—such as mirrors—for a telescope or reflectors for a radio wave antenna. These systems also typically include an alignment apparatus for directing the radiation sensitive component at a desired target. The alignment apparatus may also be used to make fine adjustments to the position of the radiation sensitive component to improve the image of the desired target. In general, an alignment apparatus may include the ability to accurately control the position of one part of a system relative to a second part, either to effect a relative displacement between the parts or to maintain a desired spatial relationship between the parts. One type of device used in the art for position control is the actuator mechanism. Actuator mechanisms, however, tend to be mechanical in nature and, consequently, can be heavy and unreliable.
One such actuator mechanism is an “inch worm” type linear actuator in which high resolution is achieved by directly moving an actuator armature in small steps using thermal, piezoelectric, electromagnetic, or magnetostrictive armature translators. For example, ceramics exhibiting the piezoelectric effect have been used as actuators for accurate positioning purposes, but the range of movement is small—about 5 micrometers (μm)—and relatively large voltages are required for their operation. Such armature translators can easily move the armature in nanometer range increments and can exert very large forces, since they rely on the stiffness of an expanding or contracting material. Sequential operation of paired, electromagnetic clamp assemblies and the armature translator provides a step-wise linear motion. When power is removed, the mechanism prevents further motion of the armature. Due to their weight and power requirements, however, these actuator mechanisms generally are not suitable for large, space-based telescope and antenna systems. Because of the weight constraints described above, it is desired to reduce the weight of the various components and to provide lightweight and reliable components for use in space-based telescope and antenna systems.
Terrestrial optical and antenna systems, used where gravity is significant, may also benefit from the use of lightweight reflecting systems. Any terrestrial mirror system will deform when its orientation to gravity is changed, for example, by aiming the system. The use of heavy monolithic mirrors addresses the problem by designing the mirror structure with high stiffness. However, regardless of stiffness, weight is always a factor due to deformation of the mirror structure from gravity. An approach is needed that allows the system to adapt to changing gravity loads and other factors that cause deformations.
As can be seen, there is a need for a lightweight mirror system that can provide massive light-gathering capacity, while allowing compact stowage in a spacecraft payload compartment. There is also a need for a compact and low mass actuator mechanism for terrestrial and space-based reflecting systems.